One of the results of the energy crisis of the early '70's was a renewed focus on energy efficient designs for homes and buildings. Many of the energy efficient designs developed during the 1970's and 1980's focused on the collection, storage and distribution of heat energy. Such designs often resulted in architecturally unique structures. Often the designs, especially those using solar energy, were dependent on machinery. Even passive designs required massive heat sinks and, sometimes expensive, architectural details.
In recent years a better understanding of how heat migrates out of a house or building through air leaks and underinsulated walls has led to new designs. These designs employ recently developed technology in mechanical ventilation which assures an adequate supply of fresh air to a home or office while recovering a substantial portion of the heat contained in the air vented from the house. Technological improvements in the design of super-insulated windows, in combination with high R-value walls and ceilings, together with tight construction, has resulted in a new type of energy efficient building designs. These designs lose very little heat and thus the building or house can rely on the heat supplied by the building's occupants and the appliances and lighting to supply the majority of the heat required by the building. Supplemental electrical heat or a wood stove is easily sufficient to heat the super-insulated building.
The key to the design of the super-insulated building is the use of walls and roof which allow the installation of as much as a foot or more of insulation which, in turn, produces R-values of 50 or more.
One type of system known as the Larsen truss, was designed by John Larsen, a builder in Alberta, Canada, more than a decade ago. His system, as described in an article in Fine Home Building, Spring 1994, by Jim Young at page 79, is produced with two-by-two vertical cords which had been dadoed to accept one foot squares of 3/4 inch plywood spaced every two or three feet. These trusses or similar ones such as described in the Fine Home Building article, are nailed on to the exterior walls of the house into the headers and rim joists. The trusses are then joined to each other with short lengths of two-by-threes toenailed to each other and into the cords of the trusses. To anchor each truss at the top of the walls of the house the uppermost plywood gusset is extended an inch and 1/2 and nailed to adjacent roof trusses. The use of Larsen trusses, as described in the Fine Home Building article, is quite effective. A house measuring 2,600 square feet had a heating bill of only $225.00 a year and is located in an area that experienced 8000 heating degree days per year.
Larsen trusses, while effective in producing super-insulated buildings, have some drawbacks in regard to cost and limitations imposed in the construction process. In the Fine Home Building article, the author, Jim Young, attributes almost half the cost of installing and utilizing the Larsen trusses to the additional labor used on the trusses. This labor amounted to over three times the cost of the materials used in the trusses.
An additional complication of the use of Larsen trusses is that they are in part hung from the roof trusses. Thus, before they can be installed the roof trusses must be installed. Further, the roof trusses then must be designed to support the additional overhang together with the loads imposed by the hanging trusses. In a conventional house or small office building the first floor of the building is constructed on a platform or floor constructed on joists laid on the foundation walls. The first story walls of the house are then erected on the floor. Once the walls are erected the roof trusses are positioned on the upper sill of the walls and the roof and the walls of the building are sheathed and enclosed. The use of external super-insulated trusses results in an additional step in the construction process which can add to overall costs.
Other methods of forming super-insulated walls including building two conventional walls spaced apart or using large dimensional framing timbers such as two-by-twelves requires considerable additional labor and materials.
What is needed is a system for building super-insulated homes and offices where the superinsulation is integral with the construction of the walls.